Experimental evidence points to important interindividual and sex differences in responses to drugs of abuse, stressors, and in the function of neurotransmitter systems thought to mediate those responses. Genetic variations may account for a significant proportion of that variance, possibly modifying the subsequent risk for the development of substance abuse in humans. Understanding the mechanisms linking relevant genotypic variants with behavioral phenotypes requires a systems-level approach that includes the study of intermediary brain responses. In other words, we cannot ignore that the pathway from genetics to behavior is through the brain. These intermediary endophenotypes include neurotransmitter systems directly regulated by the genotypes, as well as downstream modulatory influences. The present proposal focuses on a common functional single nucleotide polymorphism known to directly influence dopamine neurotransmission and which has been implicated in modifying responses to stress and substances of abuse. Preliminary data is presented demonstrating the differential activation of downstream, regulatory mu-opioid system responses to a pain-stress challenge as a function of this genotypic variant, resulting in significant interindividual differences in physical and psychological responses in humans. The present proposal seeks to confirm and extend these findings in a sample of healthy human volunteers selected on the basis of genotype. We are to examine the functional responses of D2 dopaminergic and mu-opioid neurotransmission with positron emission tomography and neurochemical imaging with specific radiotracers and validated quantification models. Measurements will be performed during control conditions and during an experimental challenge (moderate levels of steady sustained pain, utilized here a model of physical and psychological stress) known to activate those neurotransmitter systems. The individual phenotypic responses to this challenge will then be associated with the presence of specific genotypes and with the function of neurotransmitter systems directly and indirectly regulated by them. Linking genotypic determinants, endophenotypic neurotransmitter responses (dopaminergic and opioid neurotransmission) and their phenotypic expression provides a unique, systems-level understanding of human physiological processes. Such an understanding is essential for the elucidation of the multiple neurobiological factors involved in the individual responses to stress and the action of drugs of abuse, known to confer varying levels of risk for the development of substance abuse disorders.